Who Are Pro-Russian Rebels Implicated in Missile Strike on Airliner in Ukraine?

While top U.S. officials, including President Obama, stopped short of saying exactly who "shot down" a passenger airliner with nearly 300 people on board in Ukraine Thursday, the Ukrainian Ambassador to the U.S. today put the blame squarely on Russia and the Russian militants he says are stoking the separatist movement in his country.

Referencing audio that the Ukrainian government says captured separatist militants speaking with Russian intelligence agents about the crash, Ambassador Olexander Motsyk said the tape "proves that Russia is responsible for this tragedy" - a claim Russia denies.

"Russia created a warzone in Ukraine. Russia sends militants, sends heavy weapons, sends anti-aircraft weapons to Ukraine... and that's why Russia is responsible together with those terrorists who [shot] down the airplane," Motsyk said.

Pentagon spokesperson Rear Admiral John Kirby said today the U.S. still doesn't know "exactly who is responsible" for downing the plane, but he, other American officials and experts said the type of weapon believed to have brought down the aircraft, the Russian-made SA-11 surface-to-air missile, is too sophisticated for the rebel group to use "without some measure of support from the Russians."

Beyond simple support, American and other top Western officials have for months strongly suspected what Ukraine has claimed all along - that some major players and other well-trained troops in the separatist movement are not Ukrainians, but Russians, sent by Moscow to sow dissent.

In April, NATO Supreme Allied Commander of Europe Gen. Philip Breedlove laid out the case for the clandestine Russian invasion, saying, "It's hard to fathom that groups of armed men in masks suddenly sprang forward from the population in eastern Ukraine and systematically began to occupy government facilities."

"It's hard to fathom because it's simply not true," he wrote on NATO's website. "What is happening in eastern Ukraine is a military operation that is well planned and organized and we assess that it is being carried out at the direction of Russia."

Last month Breedlove told reporters that Russian advisors had trained separatists on advanced, vehicle-borne anti-aircraft systems, like the one believed to be responsible for downing Malaysian Airlines flight MH17 Thursday.

"These paramilitary sources that we don't talk about as much anymore certainly didn't act or behave, or were organized or resourced like some rag tag militia," Kirby said today. "Nobody's suggesting that Russian military advice and assistance hasn't somehow crossed that border. It's just unclear how much and when and who."

ABC News consultant Richard Clarke, a former White House counter-terrorism advisor, said that while there are certainly many Ukrainians fighting in the rebel movement, many of the "core" group of militants are actually former soldiers from Russia. By using those men, Clarke said, the Russian government maintains "deniability."

"They are soldiers of fortune, Rambo types who have fought in Russian wars," Clarke said. "They are people in close contact with the Russian security services, people who have apartments and homes in Moscow, and people who are probably being paid by Russian security services to be the military heart and core of the rebels... These are the dogs of war."

The Ukrainian government reportedly said one of the rebel leaders, Igor Strelkov, for instance, is a registered citizen of Moscow and a former officer in the FSB, the successor of the notorious KGB. Georgetown University professor Christopher Swift said Strelkov has had a "long standing relationship with the Russian intelligence services." The Russian Embassy in Washington, D.C. said today it did not "have any information about Mr. Strelkov."

A New York Times report in May painted a more complex picture of some troops on the ground for one rebel company in Donetsk, noting that while most of the fighters in that group were Ukrainian, many had close ties to Russia and relatives there. The company commander, the Times said, was a Ukrainian who had served in the Russian military - "a background that could make him both authentically local and a capable Russian proxy."

U.S. officials have also accused Russia of providing the separatist movement with heavy weapons, including anti-aircraft systems. While the U.S. officials didn't comment directly on the origins of the sophisticated SA-11 system, the Ukrainian security services released a video today that it claims shows an SA-11 system being trucked back towards the Russian border, missing two of its four missiles.

Today U.S. Secretary of Defense Chuck Hagel said the idea that the missile system came from Russia is stating "the obvious."

"That surface-to-air missile had to come from Russia, the training provided to operate that surface-to-air missile system likely came from Russia," Hagel told Bloomberg TV. "Whether the Russians knew specifically about this incident or not, I don't know... [But] the Russians are going to have to take some responsibility for it."

The Kremlin has long denied any military involvement in Ukraine, a line echoed by the Russian Embassy in Washington, D.C. today.

"There are no Russian military on the Ukrainian territory," the Embassy wrote to ABC News in an email. "Russia had nothing to do with the Malaysia Airlines flight."

Responding to criticism from Australian officials overnight, Russian Foreign Minister Sergey Lavrov said such criticism should be directed "to the Ukrainian authorities, who bear full responsibility for what transpired on the territory of that country."

"Those countries which actually encourage Kiev's punitive operations against its own citizens in the southeast of the country, in spite of our repeated warnings, will have to share responsibility," he said.